The cost of telephone sets has decreased to such a degree that many, perhaps most, homes are now equipped with several of them. New homes are prewired for telephones in every room, and older homes are being re-wired to strategically add telephones at various locations. All of this effort is aimed at improving convenience so that telephones can be within easy reach--no matter where individuals are located within the home, whether in the basement or the bedroom. The proliferation of telephones within the home has been encouraged by the fact that they can all share the same wire-pair. This has led to problems when more than one person wants to use the telephone at the same time. In particular, it is annoying for a person already engaged in a telephone conversation to endure the loud sounds that occur when another person begins dialing. Furthermore, there still exist sensitive persons who feel badly when they disturb others. These concerns have been addressed by telephone sets that visually display the status of the telephone line. Typically, a light emitting diode associated with a telephone set is turned ON whenever another telephone set, that shares the same line, is in an off-hook (active) state. However, detecting the off-hook state is not as easy as one might suspect.
Telephone Office equipment furnishes DC power to each telephone set over the telephone line that interconnects them. When the telephone is in an on-hook (idle) state, there is practically no DC current flow over the telephone line. When the telephone set is in the off-hook state, there is a limited amount of DC current flow which is used to power the telephone set and is detected by the Telephone Office to activate equipment to serve the telephone set. U.S. Pat. No. 4,451,707 discloses a known design for a line status circuit which uses a fixed reference voltage to determine whether the telephone set is in its on-hook or off-hook state. However, the amount of DC current flow over the telephone line is highly variable because, for example, the length of the telephone line varies considerably, so the voltage across the line is not the same among telephones in the off-hook state.
Determining the appropriate on-hook/off-hook reference voltage is further complicated by the existence of low-voltage SLC (Subscriber Loop Carrier) lines which can have an on-hook voltage of 12 volts. This is less than the oft-hook voltage of some telephones on very short telephone lines. Solutions to this problem have taken one of two approaches: (1) ignore the low-voltage SLC case since their percentage is small, or (2) provide a customer-selectable reference voltage using a switch and a simple installation procedure. In case (1), telephones connected to SLC lines will always indicate that the telephone line is in use, thus rendering the feature useless; and in case (2), there is the added cost of the switch as well as an inconvenience to the customer who must now perform another installation procedure which might not be performed correctly.
Additionally, it is known to sense line voltage transients to determine when another telephone set on the same telephone line goes off-hook. Such techniques, however, rely on voltage changes and can be "fooled" by inadvertent noise as well as the operation of a pulse dialer. Accordingly, it is desirable to determine the status of a telephone line reliably--regardless of its length or noise condition.